Why is your phone still running low on battery?

Many of today's mobile phones have battery capacities of over 4000mAh, which means they last longer (of course, some mobile phones' new chips have also done a lot of work on saving power), but the lighter the phone, the better it feels. The design concepts of different brands all have their own merits.

Mobile phones, from the original big brothers to today's smart phones, have seen a leap in battery capacity rather than an increase. However, our anxiety about batteries has not eased at all. Why can't our increasingly advanced batteries meet people's growing demand for battery life?

The history of mobile phone development is also a history of battery anxiety

In 1973, Martin Cooper, an American engineer from Motorola, invented the world's first real mobile phone. However, using this phone is more like lifting weights than making a call, as it weighs 2 kilograms.

Compared with the so-called "mobile phones" before it, the weight of Cooper's mobile phone is much lighter. You know, the "mobile phone" produced by Sweden's Ericsson Company in the 1950s weighed nearly 40 kilograms of batteries alone, which could not be used by hand.

Ten years later, the "mobile phone" we know as the "big brother" was born. As the first commercial mobile phone, it was not light, weighing almost 0.9 kilograms.

Half of the huge body of the "big brother" is battery, but its battery capacity is only 500mAh, and it takes 10 hours to fully charge. After fully charged, it can only make calls for 30 minutes, truly achieving "charging for 2 hours and talking for 5 minutes".

The battery used in the "big brother" is a nickel-cadmium battery, which was very advanced in the 1980s. As the name suggests, the nickel-cadmium battery has two poles, one end is nickel (positive electrode, 2NiOOH) and the other end is cadmium (negative electrode, Cd). The battery solution is sodium hydroxide (NaOH). When the battery is discharged, the cadmium at the negative electrode reacts with two OH- to release two electrons, and two new OH- are formed at the positive electrode to ensure that the concentration of the battery solution remains unchanged.

▲ The working principle of nickel-cadmium batteries. Image by Zhang Yuchen

Compared with the first generation of rechargeable lead-acid batteries (which have never been used in mobile phones), nickel-cadmium batteries are smaller in size, have larger currents, and can be charged and discharged 500 times without any problems. Of course, they also have many disadvantages, such as the battery is prone to heat up, the cadmium element is toxic, and it is difficult to recycle, etc., but none of these problems are as serious as the memory effect.

The so-called memory effect means that if you charge the battery before it is fully charged, the capacity will decrease. For example, if you often charge the battery when it is 50% charged, the battery will "think" that the minimum charge is 50%.

For a mobile phone that can only support half an hour of talk time, the memory effect is fatal, so fatal that you may even want to throw away this phone worth tens of thousands of dollars. The shadow of the memory effect even affects us today. When using today's lithium batteries, many people will unconsciously think that it is better to discharge the battery before charging the phone (in fact, it is just the opposite).

The commercialization of a new generation of nickel-metal hydride batteries in 1989 alleviated the power embarrassment caused by the memory effect. Nickel-metal hydride batteries are non-toxic, and although they have a memory effect, they are easy to recover, and they have a larger capacity: a mobile phone equipped with a nickel-metal hydride battery launched in 1997 has a battery capacity of 1300mAh.

However, NiMH batteries also have their own problems. They generate a lot of heat and leak a lot of electricity when not in use. Even if the phone is turned off, it cannot prevent the battery from running out of power. Therefore, NiMH batteries have been replaced by lithium batteries, the light of mobile phones, as they are gradually commercialized.

The light of mobile phone——lithium battery

In 1991, lithium-ion batteries, or lithium batteries as we often call them, entered the commercial field for the first time, and mobile phones finally achieved long-lasting and stable battery life. It can be said that without the support of lithium batteries, there would be no powerful mobile phones and various apps today.

Lithium batteries are different from nickel-cadmium batteries and nickel-metal hydride batteries. When the latter two are charging and discharging, electrons and OH- run back and forth, while nickel and other metals are stationary. In lithium batteries, lithium ions and electrons run back and forth.

Take the lithium battery we commonly use now, for example. Its negative electrode is generally a graphite sheet structure, and lithium atoms are "sandwiched" between these sheets, while the positive electrode can be various types of metal oxides, which are in a grid shape, and lithium atoms can also hide inside. The charging and discharging process of this structure of battery is actually the lithium ions escaping from the "cage" and passing through the separator in the middle of the battery, and the electrons running back and forth through the wires.

▲ The working principle of lithium battery. The lattice structure here is only for illustration. The lattice structure of different materials is different. Image by Zhang Yuchen

Compared with the other two types of batteries, lithium batteries have large capacity, no memory effect, and less heat. Apart from being expensive, they have almost no disadvantages. The long-lasting tablet computers in the memories of the post-80s generation are all thanks to lithium batteries.

The capacity of today's lithium batteries has increased several times compared to previously used lithium batteries, but the speed of this increase still cannot keep up with the pace of the times.

Today's smartphones have advanced performance and are almost a combination of camera + computer. Various large-scale software and instant messaging software consume a lot of power. Large screens and 120Hz high-definition screens will also increase power consumption. On the other hand, we are spending more and more time using our mobile phones.

Can lithium batteries be further developed to be more powerful and durable? I'm afraid it's difficult!

To improve the performance of lithium batteries, all you have to do is increase the proportion of lithium atoms. Currently, the proportion of lithium atoms in lithium-ion batteries is less than 1%. It seems that there is still a lot of room for improvement, but the safety after the improvement cannot be guaranteed. Specific examples will not be elaborated here.

Overcharging and over-discharging can easily lead to the destruction of the positive and negative electrode lattice system of the lithium battery, thus causing bulging.

So, how do you charge lithium batteries?

Lithium batteries have no memory effect, but they are afraid of over-discharge and over-charge. Excessive discharge and charge will cause some lithium atoms to be "pressed" into the positive and negative electrodes and unable to escape, resulting in a reduction in battery capacity. Therefore, lithium batteries can generally be charged when there is 20% power left, and charging can be stopped when it reaches 90%.

However, the lithium batteries in modern mobile phones all have protection circuits that will cut off the circuit when overcharged. Despite this, try to avoid charging overnight.

In addition, playing mobile games while charging will also cause the battery temperature to rise and shorten its life.

When will the battery anxiety be resolved?

The only way to fundamentally solve battery anxiety is to improve the battery.

Scientists have not given up on lithium batteries so far. Silicon and lithium metal are both considered potential materials for negative electrodes. A few years ago, scientists from Nanyang Technological University in Singapore tried to replace graphite with titanium dioxide gel, but this improvement only made charging faster, without increasing the capacity.

Lithium-sulfur batteries with sulfur as the anode are one of the technologies most likely to succeed in the short term. However, the problem of rapid degradation of the electrolyte after continuous charging in lithium-sulfur batteries is difficult to solve.

In addition, people have tried to use solid substances to replace the electrolyte in lithium batteries, so there is no risk of puncture and short circuit. However, the contact between solid substances and the positive and negative electrodes cannot be as close as that of liquid substances, and the conductivity is difficult to improve.

In addition to lithium batteries, fuel cells also provide a new possibility for the future of batteries. For example, hydrogen-powered cars are now on the market in Japan, but whether this type of battery can be used in mobile phones remains unknown.

When battery technology had not yet made a breakthrough, we could still use fast charging to solve some of our anxieties, such as an advertisement’s slogan: "Charge for 5 minutes, talk for 2 hours." At that time, the fast charging power of mobile phones was only 20W, but now mobile phone chargers can reach 100W.

Under the existing battery protection technology conditions, the damage caused by fast charging to lithium batteries can be basically ignored. In fact, many times, before your mobile phone battery is retired, you are already considering buying a new model of mobile phone!

Of course, there is also a simplest and most environmentally friendly way to solve battery anxiety, which is to put down your phone and give your eyes a good rest~

Source: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), China National Geographic BOOK

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